1. Field of the Invention
The present invention is directed to a process for making higher hydrocarbons from methane. More particularly, the present invention is directed to a process for making hydrocarbons having at least two carbon atoms in which methane and a source of oxygen are separately introduced into a melt including a molten metal iodide salt containing a catalyst comprising at least one metal selected from the group consisting of the metals of Group IB and Group VIII of the Periodic Table of the Elements.
2. Background of the Prior Art
Processes for synthesizing higher hydrocarbons, that is, hydrocarbons containing at least two carbon atoms, whether saturated or not, from methane have been a continuing aim of those skilled in the art. This is to be expected in that methane is readily available at low cost. Conversion of methane into higher hydrocarbons, which are themselves useful as intermediates in the synthesis of valuable end products, represents an obviously desirable pursuit. It is thus not surprising that many processing schemes have been devised for synthesizing higher hydrocarbons, for example, ethylene and ethane, from methane.
G. E. Keller et al., Journal of Catalysis, Vol. 73, 9-19 (1982) provides a method of synthesizing ethylene by means of the oxidative coupling of methane. In this teaching methane is converted, in a gas phase reaction, to ethylene and ethane at atmospheric pressure and at temperatures ranging from 500.degree. C. to 1000.degree. C. in the presence of a catalyst which is selected from the group consisting of an oxide of tin, lead, bismuth, antimony, tellurium, cadmium and manganese. In all cases, the catalyst is supported on alumina. It is also noted that this study indicates that Group VIII metals demonstrate very little or no catalytic activity in this reaction.
U.S. Pat. No. 4,465,893 issued to Olah discloses a process for the direct conversion of methane or natural gas, which is predominantly methane, to gasoline range hydrocarbons. In this heterogeneous gas-phase reaction, the methane or natural gas feed is condensed in the presence of a higher valence Lewis acid halide, preferably tantalum pentafluoride, niobium pentafluoride or antimony pentafluoride, supported by a carrier which is preferably fluorinated. This condensation reaction preferably occurs in the presence of an oxidizing agent such as oxygen.
A whole plurality of patents issued to Jones et al. disclose a method for converting methane to higher hydrocarbon products which comprises contacting methane and a reducible oxide of antimony, germanium, bismuth, lead, indium, manganese or tin in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; 4,443,649; and 4,444,984, respectively. In addition, U.S. Pat. No. 4,495,374, also issued to Jones et al., recites the same process generically wherein methane contacts at least one reducible oxide of at least one metal and is converted to higher hydrocarbon products.
U.S. Pat. No. 4,450,310 to Fox et al. describes a process for converting methane to olefins and hydrogen at 500.degree. C. wherein methane, in the absence of oxygen and water, is contacted with a catalyst comprising the mixed oxide of a first metal selected from lithium, sodium, potassium, rubidium, cesium and mixtures thereof, a second metal selected from beryllium, magnesium, calcium, strontium, barium and mixtures thereof and, optionally, a promoter metal selected from the group consisting of copper, rhenium, tungsten, zirconium, rhodium and mixtures thereof. This gas phase reaction preferably occurs at a temperature of between about 700.degree. C. to about 1000.degree. C.
U.S. Pat. No. 4,199,533 to Benson describes a gas phase reaction for converting methane into higher molecular weight hydrocarbons by using chlorine gas as a recycleable reactant. Thus, this reaction occurs at a temperature in the range of between 700.degree. and 1710.degree. C. without a catalyst.
U.S. Pat. No. 4,172,810 to Mitchell, III et al. sets forth regenerable catalyst-reagents and a process for using these catalyst-reagents for the conversion and oligomerization of hydrocarbons, particularly methane, at relatively low temperatures to produce products rich in ethylene, benzene or both, in admixture with other hydrocarbons. The catalyst-reagents of this invention, which can be used in this gas phase conversion of methane, include a Group VIII noble metal having an atomic number of 45 or greater, nickel or a Group IB noble metal having an atomic number of 47 or greater; a Group VIB metal oxide; and a Group IIA alkaline earth metal, composited with a suitably passivated, spinel-coded refractory support, preferably alumina. In a preferred embodiment, alumina is coated with MgAl.sub.2 O.sub.4 and impregnated with magnesium, chromium and platinum and thereafter calcined. Such a catalyst converts methane at 1300.degree. F. to a mixture containing ethylene and benzene.
In addition to the above described gas phase catalytic reactions of methane to produce higher hydrocarbons, a whole body of work has been directed to the formation of olefins and diolefins by the dehydrogenation of hydrocarbons to unsaturated hydrocarbons. In this work a gaseous hydrocarbon, preferably ethane, and oxygen are introduced into a molten salt comprising at least one metallic iodide. Such a disclosure is made in U.S. Pat. No. 3,080,435 to Nager. A survey article of this technology is also provided in Adams et al., Journal of Organic Chemistry, Vol. 42, 1-6 (1977).
Although this technology is directed to starting hydrocarbons having at least two carbon atoms, and thus excludes methane, an allegation is made in the Nager patent that methane may be converted to ethylene and acetylene by coupling. As will be seen below, the teaching of this patent does not produce ethylene from methane.
The above review of the art emphasizes the many proposed routes for forming higher hydrocarbons from methane. That so many proposals have been made for this synthesis suggests that none of these methods have met with complete commercial success. Thus, the need for a simple process to convert methane to higher hydrocarbons still remains to be discovered.